Switching devices, in particular circuit breakers, serve amongst other things for safely switching-off power in the event of a short circuit and are in this way operable to protect electric load installations. Electrical or mechanical switching units are furthermore suitable for operational manual switching of loads and also for safely disconnecting an installation from the power supply system during servicing work or while modifications are being made to the installation. Electrical switching units are often operated electromagnetically.
Switching units of this kind are technically thus high-quality electrical switching devices with integrated protection for motors, lines, transformers and generators. Such switching units are specifically used at functional locations with a relatively low switching frequency. In addition to short-circuit protection, switching units of this kind are also suitable for overload protection.
In the event of a short circuit, an electrical switching unit safely switches off an electrical installation. Therefore, this electrical switching unit provides safety protection from overload. Any conductor through which current is flowing is heated to a greater or lesser extent. The heating is in this case dependent on the ratio of the current intensity to the conductor cross section, the so-called current density. The current density should not become too great since, otherwise, the conductor insulation can be scorched by excessive heating and a fire may be triggered. In order to protect electrical installations from these damaging effects, switching units are used as overcurrent protection devices.
Circuit breakers have two release mechanisms which act independently of one another for overload and short-circuit protection. Both release mechanisms are connected in series. An electrical release that acts virtually without any time delay performs the function of protection in the event of a short circuit. In response to a short circuit, the electromagnetic release unlatches a latching mechanism of the circuit breaker without any delay. A switching armature isolates the switching piece before the short-circuit current can reach its maximum value.
Known switching units have a contact slide unit comprising a contact slide and a moveable switching piece which, in turn, has electrical contacts. Such switching units also have first contacts to an electrical line. In a switched-on state, the electrical contacts of the moveable switching piece make contact with the fixed contacts of the switching unit. In the event of a short circuit, the electrical contacts of the moveable switching piece are released from the fixed contacts, thereby interrupting the flow of electrical current as the moving switching piece is released from engagement with the fixed contacts.
In addition to their protective functions as overload and short-circuit releases as mentioned above, circuit breakers are also commonly used to switch on and switch off motors. In order to demonstrate this function, the circuit breakers must be able to switch on ten times the motor rated current according to the product standard. To be able to ensure this limit loading, the circuit breaker must close the double break of the three current paths in the form of, in each case, one moving bridge with two contact points and two fixed contact points at virtually the same time and in a step function.
To realize this functionality, the contact apparatus comprising a contact slide and a moving bridge is released by means of a manually operated mechanism in the form of an operating element, a latching mechanism and an operating chain. The release operation takes place in a so-called rapid connection operation, or more precisely a spontaneous or sudden connection operation. In this case, the three contact systems are released by a mechanism only after the latching mechanism has already been switched on. The spring store in the form of a contact load spring then determines the kinematics of the contact system during the switching-on operation.
After the bridges strike the fixed switching pieces, the contact slide accelerates until it is reflected or redirected or rebounded at a stop. The reflection and the resulting kinetic energy of the contact slide result in renewed opening of the contact system. This can lead to welding phenomena in the event of a simultaneously elevated current in the current path.